Duke Energy wants to move millions of tons of toxic coal ash stored near Mountain Island Lake to Charlotte’s airport, part of its response to a coal ash spill last month into the Dan River.

The Charlotte-based utility sent a letter on Thursday to N.C. Gov. Pat McCrory that provides a broad outline for its statewide coal ash plans. But the N.C. Department of Environment and Natural Resources blasted Duke’s response as “inadequate.”

McCrory, the former Charlotte mayor who once worked for Duke, had required the company to provide extensive details on its plans by Saturday. The four-page letter from Duke CEO Lynn Good provided specifics on some plants but said the company is still developing a comprehensive, longer-term plan for all of its sites.

“There are far too many questions left unanswered, and Duke Energy should provide the information we originally requested, including the estimated costs of cleanup, plans for the future and a detailed timeline,” DENR Secretary John Skvarla said in a statement.

The department will move forward with “mechanisms” to obtain “necessary information” and “enforce stringent timelines for fulfillment and completion of Duke Energy’s obligations to protect public health and the environment,” Skvarla added.

The Dan River spill on Feb. 2 has put pressure on McCrory to take a tough stance against Duke. Last month, he said he wants all of the company’s ash ponds statewide moved away from drinking-water sources. A federal grand jury is also investigating the spill.

In its response to McCrory, Duke said the utility will move coal ash stored at its retired Riverbend power plant near Mountain Island Lake, Charlotte’s water source. It said it would move the ash to a “lined structural fill solution or a lined landfill.” After a location is determined, the work would take up to 4 1/2 years, the utility said.

The city said Thursday that Duke has asked it to examine plans for storing the ash at Charlotte Douglas International Airport. The proposal calls for the ash to be stored in fully lined, covered areas, unlike the current open, unlined pits that house the coal ash.

The ash could then be used as “structural fill” building material for future projects – potentially saving Charlotte Douglas money on construction. The airport is working on a plan to expand with a possible fourth parallel runway.

Duke is working with a company called Charah – which has a similar project at Asheville’s airport – to find somewhere to store the ash, according to a letter sent Thursday to Mayor Patrick Cannon and the City Council.

“We believe the project will benefit the city by allowing for an environmentally sound solution and use for all the coal ash from the Riverbend Steam Station, while providing the Charlotte Airport with graded land that can be used for future development,” Duke wrote.

Charlotte Douglas officials could not immediately be reached late Thursday. The city plans to discuss the airport ash proposal in more detail Friday.

City spokesman Keith Richardson said it’s too early to say where the ash might be stored on the airport’s property. Charlotte Douglas, an independently funded city department, owns about 6,000 acres. In addition to the terminal, airfield and related buildings, Charlotte Douglas also owns large tracts to the south, west and north of the airport’s perimeter.

Charah, a Kentucky-based company, has buried millions of tons of coal ash from a former Progress Energy power station at Asheville’s airport. The ash is buried between liners and under 6 feet of dirt, according to trade magazine reports about the project, to prevent it from leaching into groundwater.

Riverbend’s two ponds have 2.7 million tons of ash left over from burning coal. The ash holds heavy metals that, in high concentrations and with years of exposure, can cause disease. Mecklenburg County has detected arsenic, an ash element, in Mountain Island Lake water near the power plant. Duke says Riverbend has not hurt the overall health of the lake.

Duke’s plans

In Thursday’s response, Duke said it’s taking the first steps in a comprehensive plan that will address 14 sites at active and retired power plants. The company said it was committed to working with the state and will continue to refine its plans, including “the design, engineering and cost estimates.”

At the Dan River plant, the company said it will permanently close the ash ponds and move ash away from the river. After securing a site, that project would take up to 2 1/2 years.

Frank Holleman, senior attorney for the Southern Environmental Law Center, applauded Duke’s pledge to move ash from its Riverbend and Dan River ponds. The center represents environmental groups that have sued or are part of state lawsuits against Duke.

“That is what we have been trying to get them to do and commit to do since late 2012 or early 2013,” Holleman said. “It’s a very good sign that they agreed to clean up the ash at those two sites, but it’s a shame that the Dan River cleanup had to happen after a disastrous ash spill.”

Holleman said the state needs to require legally binding commitments from Duke to ensure the cleanup work is done as Good pledged.

Who will pay the price tag for moving ash ponds remains a flash point. Good said last week she expects Duke’s 3.2 million North Carolina customers to pay the tab.

Democrats at the North Carolina legislature on Thursday said Duke’s shareholders – not customers – should pay for the cleanup. House and Senate Democrats on Thursday unveiled the framework of a bill they intend to introduce when the General Assembly reconvenes in May.

State Senate Leader Phil Berger, a Republican, on Thursday said Duke should not be allowed to pass the cost of the Dan River cleanup to customers. As for other ash ponds, he said Senate Rules Chairman Tom Apodaca is drafting legislation to require Duke to clean up other sites in North Carolina.